My invention relates to the field of electrophotographic copiers and, more particularly, to a system which combines the function of registration of the copy sheet with the image on the photoconductor and the function of transferring the developed image from the photoconductive surface to the copy paper.
Most electrophotographic copiers of the prior art include a drum carrying a coating of photoconductive material which is successively moved through a charging station at which a corona applies a uniform electrostatic charge to the surface, through an exposure station at which the surface is exposed to a light image of the original to result in an electrostatic latent image of the original and then through a developer station at which the latent image is subjected to the action of a developer including a carrier and particles of colored toner, which adhere to the surface over the charged areas thereof, thus to develop the image.
In at least one specific copying machine in the prior art, the sheet of paper to which the developed image is to be transferred is fed to the nip between a pair of registration rollers located adjacent to the surface of the member carrying the photoconductor prior to the arrival of the leading edge of the developed image at a location adjacent to the registration rollers. In timed sequence with the arrival of the leading edge of the developed image at a predetermined location, the registration rollers are driven to move the leading edge of the copy sheet into engagement with the drum surface in registration with the leading edge of the developed original. Next, the copy sheet and the developed image move together through a transfer station at which the developed image is transferred from the photoconductor surface to the surface of the copy sheet. Finally, the copy sheet is picked off the drum and delivered to the machine operator. Subsequently, the photoconductor surface may move through a cleaning station and, possibly, an exhaust exposure station.
Machines of the type described above incorporate a number of defects. The first of these is in the transfer operation, in the course of which the developed image is moved from the surface of the photoconductor to the surface of the copy sheet. The vast majority of electrostatic copying machines of the prior art employ a corona to effect transfer of the developed image from the surface of the photoconductor to the surface of the copy paper. This corona, which is located behind the copy sheet, is of such a polarity as to cause the toner particles making up the image to move from the surface of the photoconductor to the copy sheet. This system suffers from the defect that the contact between the surface of the paper and the surface of the photoconductor is not very good. Copy paper which is somewhat bowed or rough paper does not contact well enough to produce a good transfer and even smooth paper does not afford the uniform and complete contact which is necessary for optimum transfer of the image from the photoconductor surface to the paper. Evidence of this is the mottling which is present in solid black areas of the developed image.
As an alternative to the corona system described hereinabove, certain plain paper copiers have employed roller transfer devices in which a soft roller presses the copy paper against the surface of the photoconductor. This roller is made somewhat conductive and has a potential applied thereto which aids in transfer of the image. I have discovered that such relatively soft rollers do not provide the required pressure to deform the paper to afford as effective contact as is necessary, particularly where a relatively rough paper is being used. Not only does a system of this sort not provide contact which is as effective as is desirable but also, where a liquid developer is used the roller quickly becomes dirty as a result of contact with the photoconductive surface at the leading and trailing edges of the copy, and also beyond the edges of a relatively narrow sheet of copy paper where the machine is intended to accommodate different widths of copy paper. As the roller becomes dirty, it in turn dirties the back of the copies resulting in unsatisfactory copies.
In addition to the foregoing, as is known in the art, one of the most expensive components of most machines of the prior art is a drum which carries the photoconductive material. Space around the periphery of the drum is at a premium. That is to say, the number of elements which must be arranged around the periphery of the drum and the size of these elements in some degree dictate the diameter of the drum. Thus, if the number of elements which must be arranged around the surface of the drum can be reduced, the diameter of the drum and the resultant cost of the machine can be reduced.